Many of compounds having a 2,3-dihydrobenzofuran ring bearing a substituted amino group at the 5-position of the ring have useful physiological activities and are important starting materials for medical products. Therefore, there are wide demands for industrially advantageous processes for producing 2,3-dihydrobenzofuran derivatives having an amino group at the 5-position of the ring as precursors thereof.
Heretofore, for introducing a substituted amino group at the 5-position of a 2,3-dihydrobenzofuran ring in known synthesis methods, a 2,3-dihydrobenzofuran is constructed by using a phenol derivative wherein the substituted amino group has been introduced at the desired position as a starting material, or a 2,3-dihydrobenzofuran which is unsubstituted at the 5-position of the ring is subjected to nitration or nitrosation and then to a reducing reaction to introduce an amino group as described in Patent Document 1. Further, Patent Document 2 discloses a method for introducing an amino group into a 2,3-dihydrobenzofuran ring at the 5-position of the ring by brominating the 5-position of the 2,3-dihydrobenzofuran, reacting the resulting product with benzylamine to replace a bromo group with a benzylamino group, and subjecting to catalytic reduction for debenzylation of the benzylamino group.
On the other hand, if a diazo group could be introduced into a 5-position of a 2,3-dihydrobenzofuran ring by diazo coupling of a 2,3-dihydrobenzofuran derivative with a benzenediazonium salt, an amino group would be easily introduced into the 5-position of the 2,3-dihydrobenzofuran derivative by reducing the diazo compound obtained by the diazo coupling. However, the diazo coupling between a 2,3-dihydrobenzofuran derivative and a benzenediazonium salt does not proceed at all under conventional diazo coupling conditions. Generally, as a solvent for diazo coupling, water is used and, in some cases, a mixed solvent of water and an organic solvent is used. For example, Patent Document 3 and non-Patent Document 1 disclose the use of a mixed solvent of water and acetic acid as a solvent for diazo coupling of a 2,3-dihydrobenzofuran derivative. However, they cannot be applied to the compound of the present invention because of an extremely low yield.
Considering the value of a 5-amino-2,3-dihydrobenzofuran compound, there is a demand for a process for producing the compound in a high yield under milder conditions which are suitable for industrial production.    Patent Document 1: WO 00/34262 A    Patent Document 2: JP 2003-104981 A    Patent Document 3: WO 98/08842 A    non-Patent Document 1: Tetrahedron: Asymmetry, 10 (1999), 1521-1526,